1. Field of the Invention
This invention pertains to spread-spectrum communications, and more specifically to a method and apparatus for encoding information to be transmitted in a manner which eliminates having to synchronize the received signal using a clock-recovery circuit, consequently simplifying the implementation of spread-spectrum communications.
2. State of the art
The Federal Communications Commission opened the Industrial, Scientific and Medical (ISM) frequency bands for use by unlicensed spread-spectrum users. This action resulted in the creation of a large market in spread-spectrum communications. However, the greatest obstacle in using these frequency bands is the cost. Present technology transceivers tend to be expensive, which inhibits the use of spread-spectrum technology.
Spread-spectrum communication is an effective method of transmitting and receiving coded information which is more resistant to interference than an unencoded information stream itself. This resistance to interference is the result of the large bandwidth over which the information stream is spread, and the requirement that both the transmitter and the receiver have the same spreading code (a predetermined, fixed pattern) or signal used to spread the information over the larger bandwidth. It is essential to the operation of present spread-spectrum communication systems that the receiver be synchronized to the incoming spreading code embedded with the spread-spectrum signal. The spreading code typically contains no information itself, but rather is used to transform the information stream into a signal which is spread over a spectrum of frequencies after being modulated onto a carrier signal.
Traditional direct-sequence spread-spectrum communication is a method for transmitting digital information over a channel (transmission medium) such as air which may be subject to interfering signals. FIG. 1A is a block diagram of this traditional digital spread-spectrum transmission system 10. The information stream 12 is an input to a first combining circuit 14 which also receives as an input a radio carrier signal 16. The first circuit 14 combines the signals 14, 16 and creates a new carrier signal 18 which is transmitted to a second combining circuit 20. The new information modulated carrier signal 18 is further modulated with a spreading signal 22 in second circuit 20 which creates a new spread-spectrum signal 24 which is then transmitted by an antenna to a receiver.
The second circuit 20 creates a direct-sequence spread-spectrum signal 24 from the digital information stream 12 by superimposing a new digital sequence 22 (the spreading sequence), having a much higher clock rate, upon the information modulated carrier signal 18. The spreading sequence 22 typically consists of a fixed pattern of logic high and low values and is known to both the transmitter 10 and the receiver (not yet shown). In radio systems, the spreading signal 22 is superimposed upon the information modulated carrier signal 18 using standard digital modulation techniques in second circuit 20 as known to those skilled in the art, most commonly phase-shift keying.
Although the most common spread-spectrum transmission method is modulating information onto a carrier, and then multiplying the carrier with a wide-band spreading code as described above, it should not be surprising that the process can also be reversed. That is, it is also possible to modulate a digital information stream 12 with a spreading code 22, and then modulate the spread signal 26 onto a carrier 16 for transmission as a spread-spectrum signal 24. This method is shown in FIG. 1B Using either method, however, it is the demodulation process which is the most complex process, and consequently the most expensive.
Before illustrating the demodulation process, FIGS. 2A, 2B and 2C are provided to show illustrative waveforms found at various stages of the spread-spectrum transmitting system 10 using the method of FIG. 1B. Digital information in a data stream consists of a sequence of logic high and logic low values described for convenience by +1 and -1 values as shown in FIG. 2A. The digital information stream is represented by line 12 as it appears at the input of modulating circuit 14.
FIG. 2B shows a representative spreading signal 16. As indicated, the frequency of the spreading signal 16 is much higher relative to the information stream 12. The resulting spread signal 18 is shown in FIG. 2C. After being superimposed upon the carrier signal 20, the spread-spectrum information carrier signal 24 (not shown) is transmitted by an antenna.
Thus far, the background has only addressed generating a digital spread-spectrum signal 24, a relatively straight-forward process. In contrast, the process for receiving and demodulating the spread-spectrum signal 24 by a receiver 30 is relatively complex. The process is illustrated as a block diagram in FIG. 3. It should be remembered, however, that the spread-spectrum demodulator of FIG. 3 works with either modulating process as shown in FIGS. 1A or 1B.
FIG. 3 shows that the first step of the demodulation process is to recover the carrier by eliminating it from the received signal 28 using a carrier recovery circuit 32. This process is accomplished, for example, by execution of a coherent radio down-conversion. This is a process in which the frequency and the phase of the radio carrier (16, FIG. 1B) are estimated and then removed from the received signal 28. This down-conversion is usually implemented, for example, using a phase-locked loop or equivalent circuit 32. The signal now consists of the spread signal 18 as it existed prior to being superimposed upon a carrier 16.
The second step consists of removing the spreading signal 22 (or code) from the recovered spread signal 18 to obtain the original information stream 12. This is accomplished in a clock recovery circuit 36 by multiplying the RF demodulated signal by an exact replica of the spreading code 22. Unavoidably, the receiver 30 must have stored in a memory the same spreading code 22 being used by the transmitter 10.
One distinct drawback of the state of the art is that the clock recovery circuit 36 requires that the spreading signal replica 34 must first be aligned with the received signal 28 before extracting the information stream 12. This alignment or synchronization process is typically accomplished in the clock-recovery circuit 36. After synchronization and demodulation, the end result is the recovery of the original information stream 12. Synchronization and the use of a clock recovery circuit 36 is very undesirable because the circuitry 36 is complex, power consuming, and increases the overall cost of a receiver 30.
It should be observed that in this traditional spread-spectrum system, almost all of the receiver circuitry is found in the RF demodulating (carrier recovery circuit 32) and spreading code clock recovery circuit 36.
It would therefore be an advantage over the state of the art to provide a spread-spectrum communication system which is synchronization free. In other words, it would be an advantage to eliminate the need to align a received signal with a replica of the spreading code stored in the receiver, and thereby eliminate the receiver's clock synchronization circuitry.
A further advantage would be the implementation of a spread-spectrum system which could directly transmit analog information, instead of having to first digitize the analog information stream.